Sodium sulphur cells

ABSTRACT

In a sodium sulphur cell, the cathode current collector in the sulphur/polysulphide cathodic reactant comprises a carbon or graphite tube containing a solid metal, e.g. aluminum, core and a deformable electronic conductor, e.g. graphite felt, as a conducting interface between the carbon or graphite tube and the core. In a preferred construction, the current collector is axially located within a cylindrical electrolyte tube, the space between the electrolyte tube and graphite tube containing the sulphur/polysulphides and a graphite felt. The outer surface of the graphite tube in this case may have grooves or recesses to form a sulphur reservoir.

This application is a continuation-in-part of our copending applicationSer. No. 550,072 filed Feb. 14, 1975 now U.S. Pat. No. 3,982,957.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invenion relates to sodium-sulphur cells and is concerned moreparticularly with the construction of the cathode current collector.

2. Prior Art

Sodium-sulphur cells have a solid electrolyte of beta-alumina separatinga liquid sodium metal anode from a liquid cathodic reactant whichincludes sulphur. This cathodic reactant has a composition which dependson the state of charge or discharge of the cell. As the cell discharges,sodium ions pass through the electrolyte into the cathodic reactant tocombine with negatively charged sulphide ions to form sodiumpolysulphides; the cathodic reactant however is commonly referred to asthe sulphur electrode. It is necessary to inject and extract electronsfrom the sulphur electrode and this is done by means of a porousconductive body, such as a graphite or carbon felt matrix. The porousconductor acts as a large-area electrode surface at which electrons canbe supplied to or removed from the sulphur or polysulphides during thecell reaction. As the electrical conductivity of such a porous materialis low, a current collector has to be electrically connected to thismatrix to enable an external circuit to be connected to the cathode ofthe cell.

The present invention is concerned more particularly with this cathodecurrent collector. Under the electrochemical conditions prevailing inthe sulphur electrode, even stainless steel is subject to corrosion.Heretofore however stainless steel has been considered to be the bestmaterial to employ. The corrosion of the steel has several deleteriouseffects. For example, sulphur which would otherwise be available forreaction in the cell is consumed in the formation of corrosion products.If the cell capacity is controlled by the sulphur, then such consumptionof sulphur reduces the cell capacity.

BRIEF SUMMARY OF THE INVENTION

According to one aspect of this invention, in a sodium sulphur cell,there is provided a current collector in contact with the cathodereactant and formed of an impermeable carbon or graphite tube containinga deformable electric conductor extending over and in contact with theinternal surface of the tube. The deformable conductor may be a graphitefelt or steel wool and, in this case, a conductive solid metal core isprovided within the carbon or graphite tube, the deformable conductorforming a conductive interface between the inner surface of the graphitetube and the core.

It will be seen that, by this construction, the only material in contactwith the cathodic reactant is graphite. Carbon is not significantlyattacked by sulphur or sodium polysulphides and this arrangementtherefore greatly reduces or eliminates the corrosion problems. Thecarbon or graphite tube is impermeable and thus the sulphur cannotpenetrate it; the tube may conveniently be made impermeable by pyrolyticimpregnation.

It will be seen that the cathode current collector described above is ofa composite construction. The electrical conductivity of carbon is poorand a simple carbon rod could not be used as the cathode currentcollector as its high resistivity would impair the cell performance. Byproviding a conductive member in the graphite tube in the form of ametal rod with a suitable interface between the rod and the tube, theinternal surface of the carbon or graphite tube is connected directly toa low resistance electrical path for the cathode current of the cell.The current path through the graphite tube is therefore merely throughthe thickness of the tube and not along the length of the tube. Theproblem of the resistivity of carbon is thus overcome giving thereby aform of cathode collector having high conductivity and good corrosionresistance while permitting of easy manufacture.

It would not generally be possible, in the conditions of a sodiumsulphur cell, to make use of a cathode current collector comprising asolid metal rod with a coating of carbon. During the heating and coolingoperations involved in manufacturing, filling and operating a cell, thematerials of such a composite may undergo different rates of thermalexpansion and the resulting stresses will tend to cause any coating toseparate from the substrate metal. The provision of the other deformableinterface overcomes these problems. The core may be made of anyconvenient metal of good electrical conductivity.

The external electric connection to the current collector may be made bymeans of a clamp collector around the carbon or graphite tube or bymeans of a connection to the core within the tube, for example throughan end plug for sealing the carbon or graphite tube.

The composite electrode must have a sheath which is impervious topenetration by sulphur or polysulphides so that the core is notchemically or electrochemically attacked. The graphite sheath may alsobe rendered impervious by filling the open pores with a resin or metalor any material that is solid at the operating temperature of the cell.The filler material need not be electrically conducting although it maybe. However the integrity of such a tube is governed by the relativethermal expansion coefficients of the constituent materials. It ispreferred therefore to make the carbon or graphite sheath impervious byfilling the pores with a deposit of pyrolytic carbon or graphite. Thepyrolytic deposit may be produced by filling the pores with a resin andthen heating the tube so carbonise the resin or by heating the tube in agaseous hydrocarbon atmosphere. Such techniques for making graphiteimpervious are in themselves well-known and will not be furtherdescribed.

The cell is conveniently a tubular cell with an electrolyte tube aroundand concentric with the cathode current collector, the sulphur electrodebeing in the annular region between the electrolyte and the cathodecurrent collector and the sodium electrode being in an annular regionoutside the electrolyte and within an outer housing, which housing istypically made of stainless steel and may form the anode currentcollector. The above-described cathode current collector however may beused in other cell constructions; for example a cell might contain oneor more electrolyte tubes with the sodium electrode within theelectrolyte tube or tubes, and with the sulphur electrode outside theelectrolyte tubes; in this case one or more cathode current collectorsas described above may be provided. each comprising a carbon or graphitetube containing a solid metallic core with an interface of deformablematerial between the core and tube, these cathode current collectorsbeing disposed around the or each electrolyte tube.

During discharge of the sodium sulphur cell there is a transfer ofsodium ions from the sodium electrode through the solid electrolyte tothe sulphur electrode where sodium polysulphides are formed. The volumeof material in the sulphur electrode therefore increases duringdischarge of the cell. When the cell is fully charged it is thereforenecessary that the sulphur electrode is only partly filled with sulphur;typically it is about two thirds full. The cell has to have provisionfor accommodating the increased volume of the cathode reactant and, in atubular cell, this may be done by providing a sulphur reservoir at oneend of the cell. As previously explained, because of the relatively lowelectrical conductivity of the sulphur material, it is a known practicein sodium sulphur cells to put a porous conductive body, for example agraphite felt, in the sulphur between the electrolyte and the cathodecurrent collector. To minimise the resistance of the cell however thepath length through the sulphur from the electrolyte to the currentcollector has to be kept small. The porous material also influences thecell operation by controlling the flow of liquid sulphur and sulphidesand, as a capillary medium, it can influence the location of thesereactants in the electrode. This is particularly important during cellrecharge when sodium sulphide must maintain contact with the electrolytedespite an increasing predominance of sulphur content and voidage in theelectrode material. The conflicting requirements lead to a compromise.Typically, in a tubular cell, the volume of porous material accommodatesonly the initial charge of sulphur, leaving the expansion space free.More efficient discharge would be aided by a large felt volume, butbetter mass flow on recharge would be aided by a small felt volume.

Using the cathode current collector of the present invention having animpermeable carbon graphite tube, the cathode current collector beinglocated inside an electrolyte tube, the external surface of the carbonor graphite tube may be provided with grooves or recesses. Typicallythese grooves or recesses cover about a third to one half of the surfaceof the tube. They should have a width such as to allow free access ofliquids into and out of the graphite felt provided in a known waybetween the current collector and the electrolyte surface; the width ofthe grooves or recesses however is made small enough to ensure that noparts of the electrolyte are too remote from the current collector foreffective use. Typically grooves with a width of about 2 mm. areemployed. The maximum width is comparable with the felt thickness thatis to say the spacing between the electrolyte tube and the currentcollector. It will be seen that with this arrangement, these groovesprovide a space to receive the increased volume of sulphide material ondischarge of the cell. However, even when the cell is fully charged, theinner surface of the electrolyte tube is always sufficiently close to anungrooved portion of the carbon or graphite tube of the currentcollector to give effective use of the whole surface area of theelectrolyte. As the cell discharges, the sulphur material fills thegrooves or recesses; by this construction it is thus possible to reducethe size of or eliminate any need for a sulphur reservoir at one end ofthe cell.

The grooves conveniently are longitudinal if the graphite tube is madeby extrusion. With moulded tubes, other patterns, e.g. circumferentialor helical or a criss-cross pattern, are readily possible.

The carbon or graphite felt in the sulphur can be attached mechanicallyor bonded to the carbon or graphite tube of the cathode currentcollector. Such bonding can be made with pyrolytic carbon or graphitebefore assembly of the cell; pyrolytic bonding can be carried out bypreliminary adhesion with organic resins followed by carbonisingtreatment or by forming the carbon bond by pyrolysis of gaseoushydrocarbons. Furthermore, the porous felt itself may be incorporated asan uncarbonised precursor which is fired with the carbon or graphitetube of the current collector to form the composite electrode.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying drawing is a longitudinal section through a sodiumsulphur cell.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawing there is illustrated a sodium sulphur cellhaving a solid electrolyte tube 10 formed of beta alumina whichelectrolyte tube separates a sulphur electrode 11 on the inside of thetube from a sodium electrode 12 on the outside of the tube. The tube 10is closed at its lower end and, in a typical construction, is 22 cm.long and has an internal diameter of 2.2 cm. The thickness of the tubewall is from 1 to 2 mm. Beta alumina is a ceramic material containingalumina and sodium oxide and possibly small quantities of othermaterials such as magnesium oxide or lithium oxide. It permits of thepassage of sodium ions and hence is used as a solid electrolyte toseparate the sulphur and sodium electrodes in a sodium sulphur cell. Thetube 10, in the construction shown in FIG. 1, is surrounded by astainless steel housing 13 which is of cylindrical form. The lower partof the housing, as shown at 14, has an internal diameter about 1 to 3mm. greater than the external diameter of the tube 10. The narrowannular region between this portion of the housing and the tube 10contains sodium which is molten during operation of the cell, the sodium15 in this region constituting the sodium electrode. Above this narrowannular region, the steel housing has an increased diameter for a lengthof approximately 5 cm. as shown at 16; this enlarged diameter portioncontains sodium and forms a sodium reservoir 17. Electrical connectionto the sodium electrode is effected by means of a terminal 18 on an endclosure plate 19 closing the lower end of the steel housing 14. The tube10 is located in the housing 13 by projections on the housing or by wirespot-welded thereto or by putting a porous wick material between thehousing and the electrolyte tube.

Within the electrolyte tube 10 is the sulphur electrode and a cathodecurrent collector extending into the sulphur. The current collectorcomprises a graphite tube 21 which is formed of graphite madeimpermeable by impregnation with pyrolytic carbon so as to make the tubematerial impervious to penetration by liquid. The graphite tube 21 isclosed at its lower end and has an internal diameter of about 8 to 10mm. in this particular example. Within the graphite tube is a solidmetal rod 22 forming an electrically conductive core. This core may bemade of an electrically conductive metal and, in this particular exampleis aluminium although other electrically conductive materials such ascopper or iron might be employed. The metal rod 22 has a diameter about1 mm. less than the internal diameter of the tube so as to leave anarrow annular space within the tube 21. This space is filled with adeformable electronic conductor 23 which can be any electricallyconductive material which does not react with the graphite tube 21 orthe metal rod 22. This deformable conductor 23, in accordance with thepresent invention, is graphite felt.

The conductor 23 effects electrical contact between the internal surfaceof the graphite tube 21 and the metal rod 22; the rod 22 and conductor23 form a low resistance electrical path so reducing the electricalresistance to current flow along the length of the cathode currentcollector.

Between the outer surface of the graphite tube 21 and the inner surfaceof the electrolyte tube 10 is an annular region containing the sulphurand sulphide materials forming the cathode of the cell. The greater partof this region contains a loose packing of graphite felt 25 or similarmaterial to increase the conductivity of the sulphur/sulphide materialforming the electrode. The top part of the annular region between theelectrolyte and cathode current collector at 26 is shown in the drawingwithout any sulphide material. This region forms the sulphur/sulphidereservoir. As the cell is discharged, sodium passes through theelectrolyte tube 10 to form sodium polysulphides in the cathode regionhence increasing the volume of material forming the cathode. The region26 serves to accommodate this increased volume of cathodic material asthe cell discharges.

Current to and from the cathode passes through the cathode collectorconstituted by the graphite tube 21 with its core 22 and the conductor23. An electrical terminal 29 is clamped to the outside of the graphitetube. Alternatively a plug 30 forming a top closure for this tube may beelectrically connected to the core rod 22 and used as the positiveterminal. In the embodiment shown, however, the cathode terminal 29 isclamped to the outside of the graphite tube 21 and the space above thetop of the core rod forms a reservoir 31 which allows for thermalexpansion of the core when the cell is heated to the operatingtemperature. The plug 30 is threaded into the top of the graphite tube21 to form a closure with a gasket 32 to ensure a tight seal.

A seal 35 isolates the sulphur electrode from the external atmospherewhere the current collector emerges from the sulphur electrode. Thisseal comprises an 0-ring 36 of elastomeric material and metal packing 37held in place by a threaded member 38 which engages the threads on anannular element 39 extending across the top of the sulphur region. Theelement 39 has a downwardly dependent flange 40 extending around the topend of the electrolyte tube 10 and engaging an annular seal 41 whichforms a seal between the sodium and sulphur electrodes and between thetwo electrodes and external atmosphere. The annular seal 41 lies withinan annular metal element 42 and is compressed between the bottom of theflange 40 and the upper surface of an insulating washer 43, which washerseats on a flat sealing gasket 44 on the upper surface of aninwardly-directed rim 45 around the top of the housing 13. Theinsulating washer 43 is required in this embodiment because graphite isused for the packing 41; since graphite is an electrical conductor, theseal has to incorporate an insulator constituted by member 43 as well asthe further sealing gasket 44. The seal is held in position by a cap 46which has a flange 47 engaging an external thread 48 in the rim 45. Amica washer 49 insulates the cap from member 38. It will be noted thatthe seal is not contacted by the liquid cell reactants or reactionproducts in this construction so long as the cell is in the uprightposition illustrated.

We claim:
 1. In a sodium sulphur cell having a current collector incontact with the cathodic reactant, the improvement wherein said currentcollector is formed of an impermeable carbon or graphite tube, aconductive solid metal core within the carbon or graphite tube, andgraphite felt around said core constituting a deformable conductiveinterface between the surface of the core and the internal surface ofthe tube.
 2. In a sodium sulphur cell having a sulphur/polysulphidematerial forming a cathodic reactant and a cathode current collectorextending into said cathodic reactant the improvement wherein saidcurrent collector is formed of an impermeable carbon or graphite tubecontaining a solid metal core and graphite felt around the core andextending over the internal surface of the carbon or graphite tube tothereby form a conductive interface between the core and the currentcollector tube.
 3. A sodium sulphur cell comprising a housing, a tubularsolid electrolyte within said housing, a sodium electrode sealed in anannular region around said electrolyte tube, a sulphur electrode withinsaid electrolyte tube and a cathode current collector located axiallywithin the tubular electrolyte, the sulphur electrode being in theannular region between the cathode current collector and theelectrolyte, said current collector comprising an impermeable carbon orgraphite tube, a solid metal core in the tube, and a deformable fibrouselectronic conductor in the annular space between the tube and the coreto constitute an electronic current carrying interfaces between theinner surface of the tube and the core.
 4. A sodium-sulphur cell asclaimed in claim 3 wherein the deformable fibrous electronic conductorcomprises graphite felt.
 5. A sodium sulphur cell as claimed in claim 3wherein the carbon or graphite tube is rendered impervious by pyrolyticimpregnation.
 6. A sodium sulphur cell comprising an outer housing ofgenerally tubular form closed at one end, an electrolyte tube ofbeta-alumina ceramic closed at one end axially located within thehousing, sodium in the annular region between the electrolyte tube andthe housing, a cathode current collector extending axially in theelectrolyte tube, which cathode current collector comprises a carbon orgraphite tube closed at one end and containing a solid conductive metalcore with graphite felt in the annular region between the core and theinner surface of the carbon or graphite tube to form an electronicallyconductive interface between the core and the inner face of the carbonor graphite tube, a porous carbon or graphite felt between theelectrolyte tube and the cathode current collector, a cathodic reactantmaterial including sulphur impregnating said felt, sealing means sealingthe housing to the electrolyte tube to seal the sodium containing regionand sealing the electrolyte tube to the carbon or graphite tube to sealthe sulphur containing region and further sealing means sealing the openend of the carbon or graphite tube.
 7. In a sodium-sulphur cell having acurrent collector in contact with a cathodic reactant, the improvementwherein said current collector is formed of a tube of electricallyconductive impermeable material chemically and electro-chemically inertto the cathodic reactant, a core within said tube and extending alongthe length thereof, said core being of a material having an electricalconductivity greater than that of the tube, and a deformable fibrouselectronically conductive material in the annulus between the core andthe inner surface of the tube and extending along the length of thatannulus to constitute an electrically conductive interface between thecore and the inner face of the tube.